With the demand on a high-speed, large-volume data communication system for processing and transmitting a variety of information such as radio data as well as providing voice-oriented services, there is a need for developing a technique for transmitting large-volume data through the wireless communication network whose capacity is similar to that of the wired communication network. Therefore, an error handling method is essential for minimizing data loss and for increasing system transmission efficiency.
Examples of the error handling method include a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. In the FEC scheme, a receiver corrects an error by appending an extra error correction code to information bits. In the ARQ scheme, when a received signal has an error, a transmitter corrects the error by retransmitting data. A hybrid ARQ (HARQ) scheme is a combination of the FEC scheme and the ARQ scheme. According to the HARQ scheme, performance is enhanced by confirming whether data received by the physical layer contains an error that cannot be decoded and requesting retransmission if there is an error.
The HARQ operation is performed by at least one HARQ entity included in a user equipment (UE) or a base station (BS). The HARQ entity allows continuous data transmission while waiting for the feedback (i.e., the ACK signal or the NACK signal) on successful or unsuccessful reception of previous data transmission. In a downlink transmission, the UE receives resource assignment information from the BS, and the HARQ entity in the BS performs a HARQ process indicated by an HARQ information. To support the HARQ entity, the BS may operate a plurality of parallel HARQ processes.
Hereinafter, a downlink denotes a communication link from the BS to the UE, and an uplink denotes a communication link from the UE to the BS. The downlink transmission and the HARQ operation are performed in the following manner. The BS transmits downlink assignment to the UE through a physical downlink control channel (PDCCH). Then, according to the downlink assignment, the UE receives downlink data from the BS through a physical downlink shared channel (PDSCH). When the UE receives the downlink data from the BS, the UE transmits an ACK/NACK signal to the BS through a control channel (i.e., a physical uplink control channel (PUCCH)). If no error is detected in the received data, the ACK/NACK signal is an ACK signal. If an error is detected in the received data, the ACK/NACK signal is a NACK signal. When the BS receives NACK signal, the BS may retransmit the data to the UE. The downlink data here can be referred to as a codeword or a transport block.
Resources such as frequency, time, code and space, should be distinguished between downlink transmission and uplink transmission, and there needs to a scheme by which downlink resources and uplink resources are not overlapped. The scheme is called duplexing. The duplexing is classified into frequency division duplexing (FDD), in which the uplink and downlink are identified according to frequencies, and time division duplexing (TDD) in which the uplink and downlink are identified according to times.
In the FDD, frequencies having the same magnitude are symmetrically allocated in the uplink and downlink. The FDD has been widely used due to its structure suitable for symmetric services (e.g., voice calls). In recent years, however, researches on the TDD have actively been conducted due to its structure suitable for asymmetric services (e.g., Internet services). In the FDD, since the uplink and the downlink are identified in the frequency domain, seamless data transmission can be achieved between a base station (BS) and a user equipment (EU) in the time domain for each link.
The TDD is suitable for the asymmetric services since time slots each having a different ratio can be allocated for the uplink and downlink. In addition, a channel condition is almost constant in the uplink and downlink since data is transmitted and received in the same frequency band in the uplink and downlink. Therefore, the channel condition can be immediately estimated when a signal is received. Accordingly, the TDD is suitable for an array antenna technique.
In the TDD, an entire frequency band is used for uplink or downlink, and the uplink and downlink are identified in the time domain. Thus, the frequency band is used for the uplink for a certain period of time and is used for the downlink for the remaining periods of time, thereby disabling simultaneous data transmission/reception between the BS and the UE. If the uplink and downlink are alternately allocated with the same period of time, the BS does not have to inform whether a specific time point is used for uplink transmission or downlink transmission.
In FDD as well as in TDD, uplink resources are more limited than downlink resources. Because there are not enough uplink resources for uplink feedback on downlink data reception. For example, 4 resource blocks are used for downlink transmission and 1 resource block for uplink transmission respectively. If a UE receives data from a BS using the 4 resource blocks, the UE only have 1 resource block to use to transmit an ACK/NACK signal for the data reception. This lack of uplink resource makes HARQ more difficult because the BS should decide whether to transmit a new data or retransmit the old data with relatively small amount of uplink feedback information comparing with the downlink data.
A method of performing HARQ by effectively using limited uplink resource for uplink feedback is needed.